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ARCHIVED - Earth and Starquakes

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February 05, 2005— Ottawa, Ontario

During the Christmas Holidays there was a large earthquake off the coast of Sumatra. The resulting tsunami caused devastation and loss of life on a scale that is hard to comprehend. Recovery will take a long time. The earthquake had another, less catastrophic consequence; it slightly affected the Earth's rate of rotation.

The Earth's surface is divided into large, rocky plates, which move around on the more fluid material beneath. Near Sumatra, two of these plates are colliding. The Australia plate is moving northwards, hitting the Asia plate, and then sliding under it. The energies released in this process drive a multitude of volcanoes, including Krakatoa, which exploded in 1883, with disastrous consequences. Sometimes the plates jam, and the huge forces pushing them compress and deform the rocks, storing enormous amounts of energy. Finally something snaps, the stresses relax catastrophically and there is an earthquake. Events like this result in the sudden movement of billions of tonnes of rock. To visualize what happens then, think of a spinning ice skater.

One of the most spectacular sights is to see a skater spin with her arms outstretched, and then pull in her arms, and without making any other move, spin a lot faster. What she is doing is exploiting the principle that angular momentum is conserved.

View of the Moon from Earth

Angular momentum is related to three things: the shape of a body, how much mass it has, and how fast it's rotating. The first two things we lump together in a quantity we call the "moment of inertia". By pulling in her arms, the skater changes her shape, and consequently reduces her moment of inertia. Since her angular momentum has to stay the same, she has to rotate faster to compensate. The movement of all that rock during the earthquake changed the distribution of material in the Earth sufficiently to alter its moment of inertia. To keep the angular momentum constant, our planet changed its rotation speed. There are other things that vary the rate at which our planet rotates, but that will need another article.

Neutron stars are the solid, collapsed cores of dead stars. They weigh as much as a star, but are only about 20 km in diameter. Our Sun has a diameter of about 1.5 million km, and rotates once every 27 days or so. If it were to collapse into a neutron star, its moment of inertia would become much smaller, so the Sun would therefore rotate faster, about 1000 times a second! You might think that such a thing would fly apart, but these objects are held together by their extremely strong gravity. These stars often give off narrow beams of radio waves, and whenever one of those beams points in our direction we see a pulse of radio waves, very much like the pulse of light we see from the rotating light in a lighthouse. By timing the pulses we can measure the speed of rotation, and the rate at which the rotation is slowing down. When there is a starquake on one of these stars the rotation rate speeds up by a detectable amount. The energy released in these starquakes would dwarf even the Christmas earthquake into insignificance.